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<br />4_2 Floodways <br /> <br />Encroachment on flood plains, such as artificial fill, reduces the <br />flood-carrying capacity and increases flood heights, thus increasing <br />flood hazards in areas beyond the encroachment itself. One aspect <br />of flood plain management involves balancing the economic gain from <br />flood plain development against the resulting increase in flood <br />hazard. For purposes of the National Flood Insur~uce Program, the <br />concept of a floodway is used as a tool to assist local communities <br />in this aspect of flood plain management. Under this concept, the <br />area of the lOa-year flood is divided into a floodway and a floodway <br />fringe. The floodway is the channel of a stream, plus any adj~cent <br />flood plain areas, that must be kept free of encro~chment in order <br />that the IOO-year flood be carried without substantial increases in <br />flood heights. As minimum standards, the Federal Insurance Adminis- <br />tration limits such increases in flood heights to 1.0 foot, provided <br />that hazardous velocities are not produced. <br /> <br />The floodway for the Little Thompson River was computed on the <br />basis of equal conveyance reduction from each side of the flood plain. <br /> <br />The results of these computations are tabulated at selected cross <br />sections for each stream segment for which a floodway is computed <br />(Table 2)_ <br /> <br />AS shown on the Flood Boundary and Floodway Map (Exhlbit 2), the <br />floodway boundar~es were determined at cross sections; between cross <br />sections, the boundaries were interpolated. In cases where the <br />floodway and laO-year flood boundaries are close together, only the <br />floodway boun&ary has been shown. <br /> <br />The area between the floodway and the boundary of the lOO-yefrr flood <br />is termed the floodway fringe. The floodway fringe thus encompasses <br />the portion of the flood plain that could be completely obstructed <br />without increasing the water-surface elevation of the IOO-year flood <br />more than 1.0 foot at any point. Typical relationships between the <br />floodway and the floodway fringe and their significance to flood <br />plain development are shown in Figure 2. <br /> <br />6 <br />